1. Field of the Invention
This invention relates generally to the plastic molding of objects having a plurality of protruding metallic inserts, and more specifically, to a leadframe suitable for the plastic encapsulation of semiconductor devices.
2. Description of the Prior Art
During the process of encapsulating in plastic, a semiconductor device (or integrated circuit) is assembled on a metallic leadframe which facilitates the making of external electrical connections. The device and the leadframe are then inserted into a mold cavity while the leads extend outside of the cavity. The mold is heated and the plastic is injected into the mold in liquid or semi-liquid form under very high pressure. Due to its fluidity, the plastic material runs out of the mold through any crevices where the sealing is imperfect. In the past, to avoid leakage between the leads and the consequent flash, the mold has been castellated. This, however, increases the costs of the mold and leads to further sealing problems where the thickness of the leadframe stock is variable. Another technique used to reduce the flash caused by leakage between the metallic leads is to employ dam-bars which are incorporated into the leadframe. Dam-bars comprise a portion of the leadframe extending between leads so that mold closure may be effected without castellations. Subsequent to the molding operation, the dam-bars are punched out. This punching out process dictates that the dam-bars must be sufficiently far from the package to allow their removal without damaging the package. Since the mold compound is present at the dam-bar and the space between the leads is small, the punches used to remove the dam-bars are fragile and have a limited liftetime. Two other problems inherent to the punching out process are that the leads can be damaged by the punches and that mold compound is left on the leads which may foul or damage any further equipment which handles the device. Technologic progress has led to ever increasing lead counts and smaller packages which in turn has led to decreasing space between the leads. Therefore, smaller and more fragile dam-bar punches are required and the possibility of lead damage and unremoved flash is increased.
Thus a need exists for an encapsulation process, including an improved leadframe, which will reduce or eliminate the flash that results from the molding operation, and, which will mitigate the requirements of the trim and form operation subsequent to the molding process.